In Evolution’s Race, Horseshoe Crabs Took a Slower Pace

Horseshoe crabs look as if they belong in another era. Their rounded cephalic shields recall the long-extinct trilobites, and the arrangement of legs and book gills beneath their protective carapaces certainly give them an archaic appearance. No wonder people mistakenly call them “living fossils.”

Three years ago I took a weekend trip down to Delaware to catch the arthropods in the middle of their annual spawning. My wife and I walked along the beach of Prime Hook National Wildlife Refuge and waited for the scores of horseshoe crabs to turn up, but we only spotted a few isolated pairs in the act of coupling. Even so, all that waiting gave us plenty of time to stroll along the tideline and look at some of the horseshoe crabs that didn’t catch the last wave out. Their cracked and picked-over bodies were everywhere, and I couldn’t help but wonder how long the horseshoe crabs might keep this mating ritual going. Perhaps, at some distant time, another intelligent being will take in a similar site and marvel at apparently how little horseshoe crabs have changed.

But the idea that horseshoe crabs have not changed at all in millions of years is a pernicious myth. These are not creatures that evolution has left behind. The modern species I saw strewn across the Delaware beach – Limulus polyphemus – is not found in the fossil record, and the genus to which the Atlantic horseshoe crab belongs has only had a tenure of about 20 million years. The fossil record for horseshoe crabs as a group is even deeper. While their exact origin has been difficult to pin down, the record of horseshoe crabs goes back to the early days of a group called the Xiphosurida during the Cambrian over 510 million years ago. Since that time, as paleontologist Don Prothero pointed out in his book Evolution: What the Fossils Say and Why it Matters, there have been a number of forms which deviated from the image of horseshoe crabs as we know them now. Prothero picked out the boomerang-like Austrolimulus and the “double-button” Liomesaspis as two examples of disparate body plans within the group and – just as with crocodiles and other sorts of so-called “living fossils” – there was historically greater variation within the group than what we observe today.

Still, the horseshoe crab Mesolimulus preserved in the roughly 150 million year old Jurassic limestone of Germany looks quite similar to modern-day Limulus. Among some lineages of horseshoe crabs, at least, there has been relatively little change since the time of the dinosaurs Allosaurus and Stegosaurus. This apparent lack of change once perplexed Victorian naturalists such as Thomas Henry Huxley. Though he was Charles Darwin’s most ardent public defender, Huxley did not entirely agree with his friend about the nature of evolution and the fossil record. Huxley was bothered by the fact that Darwin did not allow for big jumps – large-scale mutations – in the evolutionary process, and the occurrence of “persistent types” of archaic organisms that had been altered little over vast spans of time led Huxley to posit that most evolutionary innovation took place during some portion of time that had not been preserved in the geologic record.

Eventually, though, Huxley came to understand what Darwin maintained all along. Seemingly archaic creatures such as the duckbilled platypus, lungfish, crocodiles, and the horseshoe crab had not changed very much from their prehistoric forerunners because they were able to find a cozy spot, seemingly free of competitive pressures that would require them to change. Just as natural selection accounted for major evolutionary changes, the theory also explained why some lineages might persist with few alterations. Huxley explained this during his 1876 tour of the United States, later printed in American Addresses:

[G]ranting the existence of the tendency to the production of variation; then, whether the variations which are produced shall survive and supplant the parent or whether the parent form shall survive and supplant the variations, is a matter which depends entirely on those conditions which give rise to the struggle for existence. … [In such a case], there will be no progression, no change of structure, through any imaginable series of ages. … Thus the existence of these persistent types, as I have termed them, is no real obstacle in the way of the theory of evolution.

Huxley was thinking in terms of derived species competing with their parent species and the parents winning out, but the core of his argument was that the exigencies of the environment would determine what variations would be favored. If conditions remained relatively constant, then there was little pressure to affect change – if it ain’t broke, don’t fix it. That’s why a newly-discovered Cretaceous horseshoe crab looks so familiar.

Paleontologists Rodney Feldmann, Carrie Schweitzer, Benjamin Dattilo, and James Farlow described the new genus and species in the journal Palaeontology. They have called the creature Crenatolimulus paluxyensis, but, strangely, the arthropod itself was not fossilized. All that remained was a mold created in rock, and the activities of other invertebrates were critical in the preservation process.

The top (left) and bottom (right) of the Cretaceous horseshoe crab Crenatolimulus paluxyensis. From Feldmann et al., 2011.

The fossil horseshoe crab was discovered in the bed of the Paluxy River near Glen Rose, Texas. Fossil aficionados should know this place well – numerous Cretaceous dinosaur tracks have been found here, and gullible creationists have cited fabricated human tracks carved into stone from the area as evidence that humans and non-avian dinosaurs once walked together.

Regarding the new find, the fossil was found as “float” – that is, in a piece of rock which broke from its layer of origin and was transported some distance. By looking at the details of the fossil, though, Feldmann and colleagues attributed the horseshoe crab to a layer which contains the remnants of an ancient sea. Crustaceans, molluscs, echinoderms, and other marine creatures dominate these layers, and the existence of some terrestrial fossils – such as dinosaur footprints – in the same deposits have helped bolster the hypothesis that around 112 million years ago the area was once covered in shallow saltwater lagoons or similar environments.

Crenatolimulus paluxyensis is anatomically close enough to the Atlantic horseshoe crab that if someone created a cast from the fossil mold and placed the reproduction on a Delaware beach, I bet that only a sharp-eyed arthropod aficionado would be able to pick it out. One of the primary differences – from which the creature received its genus name – is the crenulated pattern of its abdomen, but otherwise the horseshoe crab looks quite similar to modern forms. This, by itself, is not especially noteworthy, but the way in which this fossil was formed is.

The fossil horseshoe crab is preserved as an exterior and interior mold. No other trace is left of the animal’s body – the actual carapace was destroyed long ago. What the fossil does preserve are traces of activity by other organisms. The interior mold shows traces left behind by small invertebrates that were likely feeding on the horseshoe crab after it died, and, in the words of the researchers, “The mould of the exterior of the cuticle is replicated entirely by a tangled mass of serpulid worm tubes.” These tube worms actually created a mold of the fossil through their home-building, and the detail they preserved was so fine that when an artificial cast was made from the fossil mold the result was “a faithful replica” of the arthropod’s carapace. The media-hyped Triassic Kraken which supposedly composed a self-portrait in ichthyosaur bones was fantasy, but this unique horseshoe crab fossil illustrates how invertebrates can sometimes inadvertently preserve the anatomy of other organisms.

Feldmann and co-authors propose that the fossil was created in a few simple steps. Soon after death, the floating larvae of tube worms began to settle upon the horseshoe crab’s carapace. The worms began building their hardened homes as they matured, and the array of tubes took on the form of the deceased arthropod. Based upon the rate at which modern tube worms create their homes, Feldmann and colleague hypothesize, the whole horseshoe crab was probably covered in a matter of days. Without the worms, we would have no idea that horseshoe crabs once crawled through the shallows of Cretaceous Texas.

Top Image: An Atlantic horseshoe crab along the shore of Delaware. Photo by the author.